FPASA BULLETIN SF 11
FIRE HAZARDS OF PROCESS HEATING
Although the incidence of fires involving process heating equipment in service is low,
associated fires or explosions may injure or kill workers, destroy property and severely
interrupt production.
Parow
The fluid level in a 45 000  furnace oil tank was low. A delay in replenishing the oil
resulted in the heating elements being exposed as the draw-off pipe was below the
element. Failure of the thermostat caused the element to overheat and ignite the product
which was in the tank. The resultant explosion severed the tank at its base and propelled it
five metres distant.
Amanzimtoti
A larger than normal accumulation of oil had collected on a ledge on the furnace and in a
drainage channel on the floor. This unignited oil emanated from the burner jets which were
set back from the blow-hole on the furnace. It is assumed a burning droplet ignited the oil
on the ledge and in the channel. The fire burned through the oil feed line and burning oil
spread over the floor. Although controlled and extinguished quickly, the intense heat
damaged control units and the roof structure.
Causes of fire
The causes of most process heating fires can be attributed to human failure such as:
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inadequate training of operators and technicians
faulty design
user carelessness
Heat utilising equipment
The most common equipment utilising process heating comprises ovens, furnaces, kilns,
forges and dip or quenching baths. These are usually heated by electricity, oil, gas steam
and solid or pulverised fuel. The heating systems usually used are:
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direct-fired when the combustion products feed into a chamber containing the
workpiece
indirect-fired when the combustion products do not feed into the chamber containing
the workpiece
heat transfer medium – where heat from electric or steam coils, etc. warms liquid, oil or
air in contact with the workpiece.
Fire hazards
The fire hazards of process heating equipment include:
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the combustibility of the construction and contents of rooms where the equipment is
located.
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the presence of flammable vapour and air mixtures occurring as a result of faulty
operation, or because the workpieces are combustible or are coated with combustible
finishes
over-heating of equipment
poor storage and conveyance of fuels used for heating.
Fire safety requirements
Although the following recommendations are ideally suited to ovens and furnaces, many of
the principles are applicable to other heat utilising equipment.
Siting and installation
Ovens and furnaces should be positioned where they present the least exposure to life and
property and should be separated from combustible stock and critical areas such as power
supply equipment, control equipment, etc. Consideration should be given to access for
maintenance, the installation of fixed fire suppression systems and manual fire-fighting.
The size of fire likely to occur as a result of over-heating or escape of fuel must be
considered. Large ovens and furnaces should be segregated from other sections of the
building by fire resistant construction elements. Basement localities must be avoided
because natural ventilation and explosion venting is complicated.
All energised or high temperature furnace components must be insulated to prevent injury
to personnel, ignition of materials being processed, or contact with any accidental fuel
leaks.
Ovens and furnaces should have individual ventilation facilities terminating on the outside
of the premises. Unless the products of combustion discharge directly into the oven, oil
and gas fired units should have separate ventilation facilities for the heater unit and the
oven. Natural draft is inadequate for effective ventilation so mechanical air supplies are
usually provided.
Where there is a possibility of overflow from salt baths, melting furnaces or heated vats,
precautions should be taken to prevent any overflow reaching combustibles. Rooms or
parts of rooms where heated metal is rolled or power forged should have an earth or fire
resistant floor for 1 m and non-combustible wall partitions and supports for a distance of 3
m above and 10 m at the sides, from any point where hot metal is handled.
Safety controls and devices
Suitable safety devices should be fitted to all ovens and furnaces where flammable or
combustible materials are processed or used as a fuel. The equipment is designed to
supervise constantly operating conditions to ensure safety and avoid fires and explosions.
This is achieved by using an adequate number of suitable “fail-safe” devices fitted to the
system. To ensure their effective operation, safety devices should be regularly tested and
well maintained. Thermostats can fail but if duplicated, reduce the possibility of
overheating occurring. Level switches which cut off the heat source in sub-surface liquid
heating systems are advocated. These are activated at pre-determined safety levels.
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Fuel and electricity supplies
Electricity
elements
design should facilitate isolation of power supplies to the heating
while the supply to control and alarm circuits is maintained. Isolation
switches should be well indicated.
Gas
should be
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town gas or LPG are normally used. Where possible gas pipes
installed so as not to be vulnerable to damage but also must be
accessible
for inspection. The main supply pipe should be fitted with a well
indicated manually operated emergency valve. All piping in the
premises
should be as short as possible and must not pass over or under heat
treatment equipment.
Oil
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installations should be fitted with:
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regulating valves to control flow
automatic fire valves for emergency fuel isolation.
These should also be able to be controlled manually
from a remote position.
Fire and explosion protection
Suitable systems to control the effects of fire include:
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waterspray systems where combustible materials in ovens could sustain
combustion
carbon dioxide, foam or halon systems which may be supplementary to but not
replace sprinklers
steam which may be used only when oven temperatures are in excess of 100°C
portable fire extinguishers installed near ovens, heaters and finishing processes
such as dip tanks
explosion vents on ovens containing flammable vapour or gas-air mixtures.
Venting ratios should be about 1 m² for every 15 m³ of oven volume where
flammable mixtures are present and 1 m² for every 30 m³ of oven volume where
no flammable mixture exists and the oven has been fitted with all relevant safety
devices.
Operator training
As the safe operation of fuel-fires equipment relies on the maintenance of proper fuel-to-air
ratios which must be released in controlled quantities and ignited by a strong ignition
source, it is obvious that the operator plays a major role. All new operators should undergo
thorough training to ensure a high level of competency.
The type of training operators should receive includes:
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combustion of air/gas mixtures
explosion hazards
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sources of ignition and ignition temperatures
atmosphere gas analysis
flammable atmosphere gas handling
toxic atmosphere gas handling
function of control and safety devices
In addition operators must have access to instructions at all times. These normally include
diagrams and procedures for light-up, shut-down, maintenance and emergency action.
References
Fire Protection Handbook NFPA
Industrial Fire Hazards Handbook NFPA
Fire Safety Data Sheet FS 6037 FPA(UK)
Published by
Fire Protection Association of Southern Africa
(Incorporated Association not for Gain0
(Reg.No. 73/00022/08)
P O Box 15467
Impala Park
1472
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